This invention relates to computer readable memory, and more particularly a writing scheme for phase change material-content addressable memory.
Content addressable memory (CAM) provides a hardware solution to the rapid search operations utilized in information networks, such as the internet. Unlike random access memory (RAM), which retrieves data from different locations in memory by a supplied address, CAMs search the entire memory for a supplied data word and returns the address and possibly associated data. This design offers a faster method for matching functions such as those performed by routers and network servers.
Most CAMs are based on volatile RAMs, such as dynamic RAM (DRAM) and static RAM (SRAM). However, the present invention is directed to CAMs utilizing phase change memory technology, or phase change memory-content addressable memory (PCM-CAM). Due to advances in non-volatile memory, the use of phase change memory provides significant advantages in scalability.
In phase change memory, information is stored in materials that can be manipulated into different phases. Each of these phases exhibit different electrical properties which can be used for storing information. The amorphous and crystalline phases are typically two phases used for bit storage (1's and 0's, respectively) since they have detectable differences in electrical resistance. Specifically, the amorphous phase has a higher resistance than the crystalline phase.
Chalcogenides are a group of materials commonly utilized as phase change material. This group of materials contain a chalcogen (Periodic Table Group 16/VIA) and another element. Selenium (Se) and tellurium (Te) are the two most common semiconductors in the group used to produce a chalcogenide when creating a phase change memory cell. An example of this would be Ge2Sb2Te5 (GST), SbTe, and In2Se3.
Advancements in non-volatile memory technology are greatly driven by the desirability of scaling down memory devices. As the dimensions of the phase change memory elements are minimized, there exists an increasing difficulty in crystallizing the phase change material to provide a low resistance state (low bit). This difficulty does not extend to producing the amorphous high resistance state (high bit). However, difficulty in crystallization provides the issue of narrowing the sensing window between a high bit and low bit.